1. Field of the Invention
The invention relates to a rod composed of polycrystalline silicon for further use in the production of a monocrystalline rod by means of a floating zone process (FZ process), and a process for the production thereof.
2. Background Art
High-purity polycrystalline silicon (polysilicon) serves as a starting material for the production of monocrystalline silicon for semiconductors according to the Czochralski process (CZ) or the floating zone process (FZ) and for the production of solar cells for photovoltaics.
Polycrystalline silicon rods are generally produced according to the Siemens process. In this case, a silicon-containing reaction gas is thermally decomposed or reduced by hydrogen and deposited as high-purity silicon on thin filament rods made of silicon, so-called thin rods or cores. Halosilanes, such as trichlorosilane, for example, are predominantly used as the silicon-containing component of the reaction gas.
The process is carried out in a deposition reactor with exclusion of oxygen. In general, two adjacent thin rods therein are connected at their free ends by a bridge to form a U-shaped carrier body. The U-shaped carrier bodies are heated to the deposition temperature by directly passing current through, and the reaction gas (a mixture of hydrogen and a silicon-containing component) is fed in.
For the production of the polycrystalline Si rods which are suitable for the production of monocrystalline Si rods by means of the FZ process, use is made of monocrystalline (usually of arbitrary crystal orientation) thin rods (filament rods). These rods are pulled in monocrystalline fashion from polycrystalline precursor rods in a separate step. The monocrystalline thin rods usually have a round (diameter 5 to 10 mm) or square (edge length 5 to 10 mm) cross section. During the deposition of silicon, the halogen-containing silicon compounds decompose and deposit as silicon on the surface of the heated thin rods. The diameter of the rods increases as deposition proceeds.
After a desired diameter has been reached, the deposition is ended and the resultant rod pairs are cooled down to room temperature. The shaped body is usually formed in U-shaped fashion, with two polycrystalline rods as legs and a bridge made of polycrystalline Si that links the legs. The legs are intergrown at their ends with the electrodes for power supply and are separated from the latter after the reaction has ended.
Since the feet and the bridges of the U-shaped bodies cannot be used for FZ refining, the yield of the finished polycrystalline rods is significantly less than 100%. The maximum length of the deposited polycrystalline Si rod is limited by the length of the thin rod used. The length of the finished polycrystalline Si rod relative to the length of the thin rod used is referred to as “length yield,” or simply, “yield”. The length of the finished polycrystalline rod is usually not more than 85% of the length of the thin rod used.
During the production of polycrystalline Si rods having a thick diameter it is often observed that the rods have cracks or break in the course of removal from the reactor or in the course of mechanical processing to form finished rods. The cracks and fractures arise in rods on account of thermal stresses caused by the temperature differences between the rod interior and the surface of the rod. The temperature differences and thus also the stresses are greater, the larger the diameter of the rod. The thermal stresses become particularly critical if the rod diameter is greater than 120 mm.
The rods afflicted with cracks or high thermal strains cannot be used for mechanical processing to form the finished polycrystalline rods and for subsequent production of the monocrystalline rods by means of the FZ process. The cracked or strained rods usually break as early as in the course of mechanical processing. If the rods withstand this treatment, they can result in serious consequences during zone refining. Since the rods are heated up to the melting point in this process, the cracked or thermally strained rods can shatter on account of additional thermal stresses. This leads to material and time losses as a result of the termination of the refining process. Furthermore, the refining apparatus can also be damaged by the rod pieces that have splintered off. Therefore, cracked and thermally strained polycrystalline silicon rods have to be sorted out prior to refining or be shortened to the defect site. Cracks in the polycrystalline Si rods can be detected visually or by means of a known method, such as e.g. sound testing or ultrasonic technology. This material exclusion once again reduces the yield. Processes conducted according to the prior art enable an average yield of the finished crack-free polycrystalline Si rods for the FZ process relative to the length of the thin rods used of not more than 50% if the rod diameter is greater than 120 mm.
The defect-free yield of the refined monocrystalline FZ silicon depends on the microstructure of the polycrystalline silicon rod used. The production of the polycrystalline silicon rods in Siemens reactors involves firstly depositing silicon on the monocrystalline thin rods in monocrystalline form. After some time, depending on the deposition conditions, the regime changes to the polycrystalline form. In this case, silicon is deposited both in the form of a finely crystalline matrix and as coarse-grained, usually acicular, monocrystalline (but often also as twins or triplets) inclusions (needle crystals) which are incorporated into the finely crystalline matrix. The needle crystals are predominantly oriented radially, wherein their longitudinal axis can exhibit <111>, <100>, or <110> orientations. The inhomogeneous microstructure has the effect that the individual crystallites, according to their size, do not melt simultaneously in the course of passing through the floating-zone melting zone. The crystallites that are unmelted owing to their size can slip through the melting zone as solid particles in the monocrystalline rod and be incorporated as unmelted particles at the solidification front of the single crystal. A defect formation is then caused at this location.
U.S. Pat. No. 5,976,481 describes avoiding cracking by means of a thermal aftertreatment of the polycrystalline Si rods in the reactor. However, the process can only avoid the formation of those cracks which arise only after the end of the deposition during the cooling down of the rods. However, cracks can already form during the deposition in the rods.
EP 0445036 describes the production of the central region of the polycrystalline Si rod under conditions such that silicon deposits there only in monocrystalline or coarsely crystalline fashion. However, this process requires monocrystalline thin rods of square cross section in a special orientation where the longitudinal axis points in the <100> direction, the production of which is very complicated and expensive. Moreover, this process requires a high temperature and low deposition rate. The lower deposition rate means that this deposition process has lower economic viability. The high deposition temperature causes high thermal stresses and thus leads to cracked rods.
U.S. Pat. No. 3,540,871, U.S. Pat. No. 4,255,463 and DE-2727305 describe processes for enabling monocrystalline deposition to be suppressed by various factors, such that from the outset only polycrystalline silicon grows. However, the methods described cannot prevent the formation of disturbing coarse monocrystalline inclusions. Moreover, the processes proposed lead to high thermal stresses in the case of thick polycrystalline silicon rods having a diameter of greater than 120 mm, such that the crack-free yield of the finished rods after mechanical processing is very low, usually lower than 40%. DE2727305 proposes how the growth of the coarsely crystalline grains can be suppressed during deposition. For this purpose, for approximately one hour the temperature (proceeding from 1100° C.) is reduced by 200° C. and the gas flow is reduced by 25% and the molar fraction of the halosilane is increased from 7-15% to 50%. This step is repeated a number of times (up to three times). This procedure additionally stresses the Si rods as a result of the constantly altered thermal stresses and leads to the visible deposition rings in the microstructure. These rings in the polycrystalline Si rods disturb the FZ refining process and cause defects in the monocrystalline FZ rod.
All of the known methods from the prior art yield either very thin polycrystalline rods or thicker rods having stresses which, upon cooling down or during further processing, lead to defects through to the total unusability of the rod.